In an existing mobile environment, terminals transmit sporadic messages of Short Message Service (SMS), Multimedia Messaging Service (MMS), etc. On the other hand, users may need a messaging service to easily exchange a conversation using an MSN messenger in a wired environment. Thus, an instant messaging service has been introduced into terminals and networks on the basis of an SIP/IP core network. According to the demand of customers and enterprises for Push To Talk (PTT) (that is, walkie-talkie), a PTT over Cellular (PoC) service and system based on the SIP/IP core network have been developed. According to rapid variation of markets for enterprises, communication businesses, etc., the desire of users to integrate and process various types of messages is increasing. In consideration of this point, a standard technology is being developed for service Converged IP Messaging (CPM) implemented through the SIP/IP core network in the standard organization called Open Mobile Alliance (OMA). The CPM is intended to allow a user to experience a seamless messaging service environment by integrating IP-based messaging services. Accordingly, the implementation of interworking between a CPM service and a non-CPM service is needed such that a service disconnection is not perceived in any messaging service. One of many features of the above-described CPM service is that multi-client environments can be provided to users.